766 HYDROGRAPHIC MANUAL PaGE 710 



A special compass rose has l)een found advantageous for making R.A.R. dead- 

 reckoning plots. This is a partial rose about 16 inches in diameter on heavy paper. 

 Each single degree is drawn for about 15° or 20° on each side of a central line extending 

 the length of the paper, and numbered according to its angular distance from the central 

 line. To use this rose a mean course or an approximate direction of the line is assumed 

 with respect to which the successive courses are plotted by differences in degrees. The 

 tracing paper on which the dead -reckoning plot is to be made is placed over the compass 

 rose and weighted down. The successive courses are then plotted on the tracing with 

 reference to the compass rose by the use of two triangles to transfer each direction to 

 the position from which it is to be plotted. In using this special rose it is best to assume 

 an average course of an even 5°. 



Dead reckoning may be plotted by log distances or by time. Each method may be 

 preferred under different conditions. If the r.p.m.'s of the engine are reasonably con- 

 stant and if a careful record of the time has been kept in the Sounding Record, the use 

 of time alone is believed preferable since this eliminates the necessity of computing 

 and checking the log intervals and generally simplifies the work. Log distances are 

 probably slightly more accurate and should be used for all comparatively long dead- 

 reckoning plots. This is only true, however, if the log is an accurate one and the survey 

 is made in waters where there is no floating debris to become attached to the log rotator 

 or log line. 



Except on the outer ends of offshore dead-reckoning lines, U-turns (where the 

 direction of the line changes 90° and soon thereafter another 90° in order to put the 

 ship on an adjacent parallel line) are not plotted unless they are necessary to fill a 

 holiday -in the surveyed area. The recorded data are rarely adequate to plot such 

 tiu-ns satisfactorily, unless a fixed position has been obtained between the two 90° 

 turns. Where it is necessary to plot a 90° turn and other evidence is lacking, approx- 

 imately two-thirds of the distance around the turn should be added to the previous 

 course and one-third of the distance to the new course. (See 3454.) 



During the survey the ship occasionally gets so far off line that an S-turn is re- 

 quired to get back on the original line. It is particularly difficult to plot the dead 

 reckoning of these turns because sufficient data relative to them are rarely recorded. 

 The data needed are the times the vessel begins to turn after each order to change 

 course, the time the vessel is steadied on the new course, and the course and time 

 steered on the intermediate course. (See 3464.) 



Each completed dead-reckoning plot should be the best possible graphic repre- 

 sentation of relative courses and distances between fixed positions. Several dead- 

 reckoning plots may be included on one sheet of tracing paper but each one should be 

 properly identified. In order to avoid confusion not more than one day's hydrography 

 should be included on one sheet. The plot should be made neatly and accurately and 

 marked for subsequent easy identification. 



All sheets of tracing paper containing dead-reckoning plots shall be transmitted 

 to the Washington Office for use in connection with the verification of the survey, 

 after which they are destroyed. 



766. Detached Positions 



The detached positions in the Sounding Record are of the utmost importance, 

 since they are usually taken to locate rocks, floating aids to navigation, least depths on 



